1. Field of the Invention
The present invention relates to magnetic resonance (MR) imaging, and more particularly relates to a method and apparatus useful for improving the quality of high resolution MR images that are sensitized to molecular diffusion.
2. Description of the Prior Art
It is well known that magnetic resonance images can be degraded by motion during the imaging scan. Bulk motion of the object to be imaged is a particular problem for MR imaging methods that are designed to detect small diffusional motions of molecules within the object. These “diffusion-weighted” images are of interest, for example, for the detection of brain lesions associated with stroke. The MR apparatus (“imager”) allows diffusion sensitization in various spatial directions, and further analysis of images made sensitive in different spatial directions yields “diffusion tensor” images or maps, which provide information about the location and direction of nerve tracts (Basser et al., U.S. Pat. No. 5,539,310). A variety of pads and clamps are used to reduce patient motion during such examinations. The effects of motion are further reduced by employing rapid “single-shot” MR methods that acquire a complete image in approximately one-tenth of a second, such as Echo Planar Imaging (“EPI”), “spiral imaging” or “projection reconstruction” (“PR”). The invention will be described using the EPI method, with the understanding that spiral, PR, or other rapid methods may be employed in an analogous fashion. In spite of these efforts to minimize bulk motion, diffusion-sensitive images are often marred by significant artifactual signal loss in portions of the field of view, and some of this signal loss is associated with mechanical vibration of the patient (Maier et al., U.S. Pat. No. 7,239,140). Magnetic resonance imagers apply time-dependent pulses of electrical current to magnetic field gradient coils during the usual scanning process. Because these current pulses occur within the static magnetic field of the apparatus, they create unwanted mechanical vibrations that are transmitted to various components of the apparatus. In particular, these vibrations may be transmitted to the patient table, and cause vibration of the patient.
In general, the large and heavy MR apparatus does not attain a stable vibration pattern instantaneously, or even during the time required for one or a small number of EPI scans. Thus, it is possible to obtain one or a small number of images free of vibrational artifacts, provided that the apparatus is vibration-free at the start of the measurement period.
The complex vibrational motions of the MR apparatus are not determined solely by the physical characteristics of said apparatus. Said motions are strongly influenced by the strength, duration, and timing of the electrical pulses (the “gradient pulses”) applied to the magnetic field gradient coils (the “gradient coils”), and by which of the (typically three) gradient coils is energized or are simultaneously energized. Thus, the characteristics of the MR “pulse sequence” used to obtain the MR image strongly affect the mechanical vibration. MR pulse sequences designed to detect molecular diffusion are especially sensitive to mechanical vibration of the patient for two reasons. First, said sequences incorporate gradient pulses that deliberately sensitize the resulting images to motion, macroscopic or microscopic. Second, said gradient pulses are deliberately of high amplitude to make the resulting images as sensitive as possible to motion. The high current amplitude gives rise to a high gradient field and a strong mechanical force (a jolt) that may cause vibration of the apparatus. Thus, diffusion sensitive pulse sequences can both create unwanted vibration and can be very sensitive to the effects of said vibration.
For the purpose of this description, it is useful to distinguish radiofrequency (RF) “excitation” pulses from “echo-forming” RF pulses. By convention, the equilibrium magnetization, which aligns parallel to the main magnetic field, is said to point in the +z direction. The purpose of an RF excitation pulse is to nutate magnetization from the z (“longitudinal”) direction fully or partially into the x-y (“transverse”) plane, where it precesses and gives rise to a detectable signal. To obtain the strongest signal, the RF excitation pulse can nutate the z magnetization by 90°, but other nutation angles are also used. In contrast, the purpose of an echo-forming RF pulse is to flip the x-y magnetization, that is, to rotate the x-y magnetization, usually by 180°, about some axis which lies in the x-y plane and passes through the origin. This gives rise to the spin echo, which is generally useful in the construction of pulse sequences. FIG. 1 illustrates a single-shot pulse sequence that acquires all of the data required to reconstruct at least one image following the application of a single RF excitation pulse 40. The five traces show the temporal relationships among the RF and gradient events, plotting relative amplitudes against time. The slice selection, readout, and phase-encoding gradients are applied along mutually orthogonal axes, for example the X, Y, and Z axes of the apparatus, while the diffusion-encoding gradient may be applied along any desired axis. Data are acquired during the oscillations of the readout gradient, only a few of which are shown. Spin echo data acquisition schemes require one (e.g., 52 in FIG. 2) or more echo-forming RF pulses, but these do not rotate additional magnetization from the z axis into the x-y plane, and do not change the single-shot nature of the pulse sequence. In contrast, multi-shot pulse sequences require two or more RF excitation pulses to acquire all the data require to reconstruct an image, and, to avoid image artifacts, precautions must be taken to ensure the consistency of the several partial data sets needed to reconstruct an image.
To acquire the data needed to reconstruct the image of a single slice, typical diffusion sensitive sequences sequentially apply two or more high amplitude gradient pulses (a “multiplet”) to sensitize (“encode”) the resulting images to diffusional motion in one particular spatial direction. Although many encoding schemes are possible, the effect of these pulses will be illustrated in a non-limiting manner by their simplest forms. The ordinary slice-selective RF excitation pulse 40 of the single-shot EPI pulse sequence is followed by a first diffusion-encoding gradient pulse of positive amplitude (41 in FIG. 1), and thereafter by a second diffusion-encoding gradient pulse of equivalent duration, but negative amplitude (43). Said first diffusion-encoding pulse changes the phases of (“dephases”) the nuclear magnets (“spins”) along one spatial direction. For stationary spins, said second diffusion-encoding pulse, which has the same spatial orientation as the first diffusion-encoding gradient pulse, but the opposite polarity, reverses (“rephrases”) the phase changes induced by the first diffusion-encoding gradient pulse. More generally, the rephasing of stationary spins occurs when the integral over both pulses is zero. Spins that have moved by diffusion or bulk motion in the selected direction during the time between the two pulses remain somewhat dephased, and the MR signal from these spins is reduced. This sensitivity to diffusion is enhanced by the use of stronger or longer diffusion-encoding gradient pulses, or by increasing the time between the dephasing and the rephasing gradient pulses. The diffusion-encoding period is followed by an EPI readout and phase encoding scheme of ordinary design (44) that acquires, for single-shot scans, all the data needed to form one two-dimensional image at one slice position in approximately 0.1 second. EPI is often performed using an echo-forming RF pulse to form a spin echo. In this case, it is efficient to apply the first diffusion-encoding gradient pulse 51 before the echo-forming RF pulse 52, and to place the second diffusion-encoding gradient pulse 53 after said RF pulse (E. O. Stejskal, and J. E. Tanner, J. Chem. Phys. 42 (1965) 288). The echo-forming RF pulse reverses the sense of the dephasing created by the first diffusion-encoding gradient, so the second diffusion-encoding gradient pulse is applied with the same polarity as the first diffusion-encoding gradient pulse, and both pulses have the same integral.
For example, to sensitize the resulting image to motion in the X spatial direction, said diffusion-encoding pulses are applied to the conventional X gradient coil. To sensitize the image to motion in a direction that is not aligned with the usual X, Y, or Z axes, simultaneous current pulses with appropriate amplitudes are applied to an appropriate combination of two or three of the conventional X, Y, and Z gradient coils. These diffusion-encoding gradient pulses are typically the strongest, or are among the strongest, gradient pulses of the pulse sequence, and will make a significant contribution to any vibration of the apparatus. To determine the spatial orientation of the diffusional motion, it is common practice to acquire additional images at the same slice position utilizing diffusion-encoding gradient pulses that sensitize said images to motion in additional distinct spatial directions. Since the spatial orientation of the gradients used for diffusion sensitization of said images is unrelated to the gradient directions used for ordinary slice selection, readout, and phase encoding, a plurality of images at the same slice position, but having different orientations of diffusion sensitization, may be acquired in separate single-shot scans, altering only the diffusion-encoding gradients. To detect and quantify the diffusion-induced reduction in signal intensity, it is common to acquire as reference an additional image at said slice position in the absence of the diffusion-encoding pulses. To obtain additional information about the diffusion process, it is also common to acquire additional images at said slice position with diffusion encoding along said distinct spatial directions, but having different sensitivities to the diffusion process (the so-called “b values”). For example, the diffusion sensitivity of the pulse sequence in FIG. 1 may be altered by changing the magnitude of gradient pulses 41 and 43 in multiplet 45 by the same factor, or by changing their durations appropriately. Each of the changes to the diffusion-encoding pulses described above is intended to produce a different image that reveals additional distinct information about the diffusing nuclear spins, and the altered diffusion parameters will be termed “distinct diffusion-encoding orientation and sensitivity combinations,” to distinguish said changes from alterations in the diffusion-encoding pulses, described below, that are deliberately designed to provide a plurality of distinct gradient pulse multiplets that maintain the same diffusion-encoding orientation and sensitivity, creating a plurality of pulse sequences that yield essentially equivalent images. These altered but equivalent pulse shapes will be termed “equivalent diffusion-encoding gradient pulse multiplets.” Having specified the desired distinct diffusion-encoding orientation and sensitivity combinations, it is clinically advantageous to acquire images at a plurality of different slice positions with each said encoding, as well as unencoded reference images at each of these slice positions. A typical diffusion-sensitized acquisition may thus generate images at a plurality of parallel slice positions, images at each of said slice positions being acquired with a plurality of diffusion-encoding directions (orientations), with a plurality of diffusion-encoding sensitivities for each of said directions, and a reference image without diffusion encoding for each slice position. Because changing the spatial direction of the diffusion-encoding is accomplished by changing the currents in two or more of the typical X, Y, and Z gradient coils, it is expected that the apparatus will experience mechanical jolts with different characteristics for each distinct diffusion-encoding orientation. It is clear that changing the diffusion sensitivity by changing the amplitude of diffusion-encoding gradient pulses will increase or decrease the associated mechanical jolts. In contrast, it is the usual practice to change from one slice position to a different parallel slice position by changing the frequency of the RF pulse or pulses, which does not alter the mechanical forces experienced by the apparatus.
Because the RF pulses that excite the nuclear spins in a specified slice of tissue have essentially no effect on the nuclear spins of a second, parallel, non-overlapping slice, “single shot” methods such as EPI can acquire a plurality of non-overlapping slices very rapidly, one right after another. However, for many medical applications, including diffusion-sensitized imaging, it is not advantageous to acquire multiple images at a single particular slice position in rapid succession, because the slice-selective RF excitation pulse (40 or 50) transiently reduces the z magnetization within the slice, diminishing the z magnetization available for the next RF excitation pulse, and thus, in succeeding images, reducing the x-y magnetization that gives rise to detectable signal. It is advantageous to allow a consistent waiting period for the nuclear spins within a particular slice of tissue to relax partially or fully back to their equilibrium magnetization before disturbing said spins with another RF excitation pulse. For single-shot imaging, all of the data for one image are acquired following a single RF excitation pulse, so failure to maintain a consistent time (TR) between the RF excitation pulses that acquire separate images at one particular slice position does not result in significant image artifacts (e.g., ghosts), but rather in undesirable variations in brightness and contrast from image to image. Such brightness differences make it difficult to evaluate the effects of diffusion. It is efficient to image a plurality of other essentially non-overlapping slice positions while waiting for the magnetization of the spins at the first slice position to relax. These considerations lead to the usual, orderly sequence of events in a single-shot diffusion-sensitive pulse sequence: all of the data are acquired for one image at a first particular slice position utilizing a first particular “distinct diffusion-encoding orientation and sensitivity combination” [vide supra], then all of the data are acquired for a second particular, non-overlapping slice position with said first particular distinct diffusion-encoding orientation and sensitivity combination, this process being repeated until data have been acquired from all desired slice positions. Thereafter, data are acquired again from the first particular slice position utilizing a second particular distinct diffusion-encoding orientation and sensitivity combination, this combination having a spatial orientation or diffusion sensitivity that differs from said first particular distinct combination. Data are then collected from the remaining slice positions in the same slice order utilizing said second particular distinct diffusion-encoding orientation and sensitivity combination. Following this pattern, data are collected for each slice position using each of the desired diffusion-encoding sensitivities or directions. In the simplest implementation of this pulse sequence, no changes are made to the timing of the diffusion-encoding pulses: the diffusion sensitivity is adjusted by gradient amplitude changes, and the orientation of the encoding is changed by a redistribution of the currents in the X, Y, and Z gradient coils without altering the vector sum gradient strength. The regular, repetitive acquisition of a “block” of distinct slice positions in a fixed spatial-temporal order ensures the desired, constant TR for all of the images, as long as the conventional pre-scans have been performed to establish a steady state of the magnetization. An advantage of this data collection scheme is that its nested, repetitive structure is easily programmed using the looping statements available in all computer languages. An example of such a scheme is shown in FIG. 3 for five distinct parallel slices at positions S1-S5, four diffusion-encoding directions D1-D4, two nonzero diffusion-encoding sensitivities (amplitudes) A1-A2, and a set of five unencoded (zero diffusion-encoding amplitude, denoted A0) slices. Each of the 45 boxes, numbered in temporal order, represents one complete single-shot image acquired with a unique combination of parameters. The time between each acquisition is constant. Each block of five slices is encoded with the same diffusion weighting, for example D1-A1 for images #01 to #05, resulting in a completely repetitive gradient pulse pattern. In this example, the unencoded slices are acquired last (image #41 to image #45). The mechanical jolting of the MR apparatus occurs as each image is acquired, typically about every 0.1 second. This period will be called the Mechanical Repetition Time or MRT to distinguish it from TR, the time between the acquisition of a single-shot image at a particular slice position and the next acquisition at the same slice position. For example, if the images in FIG. 3 are acquired, one after another, every 0.1 second, then slice S1 is acquired with a new combination of diffusion-encoding orientation and sensitivity every 0.5 second. The MRT is 0.1 second, and the TR is 0.5 second.
The mechanical jolting of the MR apparatus is a highly regular process. It is the usual practice that the single-shot acquisitions required for a plurality of slices be distributed evenly in time over the selected TR period, resulting in a steady beat of the diffusion-sensitizing gradient pulses, and a constant value of the MRT. Furthermore, the gradient pulses used to select one particular slice position do not differ from the gradient pulses used to select other parallel slice positions, so the mechanical jolts applied to the MR apparatus for each of the example five slices will be the same for the first particular distinct diffusion-encoding orientation and sensitivity combination (images #01 to #05). When this same five-slice block is again imaged with the second particular distinct diffusion-encoding orientation and sensitivity combination (images #06 to #10), the resulting mechanical jolts will have a different amplitude or direction from the jolts resulting from the first particular distinct diffusion-encoding orientation and sensitivity combination, but will maintain the same MRT.
The vibration induced in the patient is a function of the mechanical design of the MR apparatus and the amplitude, timing, and phase of the mechanical jolts created by energizing the gradient coils. The operator of the MR imager has some control over said amplitude and timing. For example, the operator may increase the TR, and thus for common sequences the MRT, to avoid an MRT period that creates a strong mechanical resonance within the MR imager and substantial image artifacts. In practice, however, there is no value of the MRT which completely eliminates mechanical vibration. Existing MR pulse sequences that acquire data in the order described by the example above usually have a sufficient number of slices, and thus a sufficient number of identical and equally-spaced mechanical jolts, to establish an undesired steady state of rhythmic motion in the patient table of the MR imager, and thus in the patient.
For multi-shot diffusion-weighted imaging, an RF excitation pulse is followed by diffusion-encoding gradient pulses, and then the collection of only part of the data required to reconstruct an image. This partial data acquisition (shot) must be repeated several times to acquire all of the data needed for an image, and the diffusion encoding must be effectively the same for each shot. Thus, when standard pulsing schemes are applied to multi-shot, multi-slice diffusion-weighted imaging, long trains of identical diffusion-encoding pulse multiplets may result. Maier et al. (U.S. Pat. No. 7,239,140) recognized that it is possible to create a plurality of diffusion-encoding gradient pulse multiplets of the same spatial orientation and diffusion sensitivity, but different mechanical properties. For example, the gradient amplitudes of pulses 51 and 53 of multiplet 55 in FIG. 2 can be negated, reversing the current flow in the associated gradient coil or coils. This reversal of polarity does not alter the spatial orientation of the diffusion encoding or the diffusion sensitivity (b value), but one may expect that the mechanical properties of the jolt resulting from such a modified pulse will differ from that of the original multiplet 55. Other possible modifications include, for example, increasing the gradient pulse duration while decreasing the amplitude, or using four encoding pulses per multiplet instead of the two shown in FIG. 2, all while maintaining the same encoding orientation and sensitivity. These modifications create pulses that are “equivalent diffusion-encoding gradient pulse multiplets” with respect to randomly diffusing spins, but not equivalent with respect to the vibrations they induce. In the absence of net motion along the encoding direction, and gradient imperfections such as eddy currents, these equivalent multiplets may be used as substitutes for some of the original multiplets without affecting the final images. The desired diffusion-weighted pulse sequence may then be played out in the usual order of slice positions and diffusion encoding orientations and sensitivities, substituting equivalent diffusion-encoding gradient pulse multiplets for an optimized or random subset of the original diffusion-encoding gradient pulse multiplets, thus altering the vibrational character of the sequence. This partial substitution may be optimized by computer simulation and monitored by a vibration sensor 32 attached to the patient table. The method is applicable to single-shot imaging. FIG. 4 shows an example of such a partial multiplet substitution applied to the same order of slices, encoding orientations and encoding sensitivities used for the single-shot example of FIG. 3. A plus sign in front of the diffusion direction indicates the same pulse multiplet used in FIG. 3, while a minus sign indicates a reversal of the gradient currents, that is, a negation of the amplitude of each gradient pulse used to create the diffusion-encoding multiplets like 45 in FIG. 1. The fully repetitive pattern of gradient pulses seen, for example, from image #01 to image #05 in FIG. 3, is broken up by irregular diffusion-encoding gradient reversals in FIG. 4. In the absence of bulk motion or gradient eddy current effects, the 45 images that result from the encoding gradients in FIG. 4 should be equivalent to those obtained from the repetitive pulse sequence in FIG. 3. The MRT has not changed, but the nature (e.g., direction) of some of the mechanical jolts will have changed, thus altering the vibrational characteristics of the apparatus.